Was recently reading how N&W raised the BP on the early A batch and the J's from 270 or so to 300. curious what all would have had to be done when doing this? Just boiler calculations to make sure still within code at the higher pressure or more involved changes?

Without opening the can of worms that is the H8, it is interesting then if straight forward, that C&O for example, didn't at least test their J3 4-8-4's and H8 at 300 psi given both had higher factor of adhesions at their as built operating pressures.